Can we do it?
Yes we can! It took me a couple of hours of twiddling to find a combination of hacks that works in Firefox. I haven't tried it yet in other browsers, but -really now- who cares about other browsers?
For my next trick, I'll write an HTML document that describes some work I've been doing in XML. The document will contain embedded XML examples; if I change the XML stylesheet, my examples will automagically display in the new format! Thus proving once again the old adage that "90% of the stuff you do with computers wouldn't be necessary if you weren't doing stuff with computers."
My usual HTML editor is Mozilla Composer, but Mozilla requires that only XHTML and not HTML can have XML embedded inside it. Composer refuses to edit XHTML documents, so I guess I'll have to use Emacs. It's not WYSIWYG but it'll do.